E. 3n
Explanation:
Of the options given, 3n represents the lowest complexity = O(3n). Since O(constant * n) is the same as O(n) this represents linear complexity.
All other given options represent a complexity higher than linear complexity.
(n) * 2n > O(n)(n2) * n > O(n)n^2 > O(n)nlogn > O(n)
This is because quadratic complexity and log-linear complexity are higher than linear.